Ochratoxin A (OCT A) has the potential to cause myelotoxicity in addition to the well-know toxic effects on the liver and kidney. Experiments reported here were designed to determine whether mice would recover from the myelotoxic effects induced by OCT A injection and secondly whether mice previously injected to OCT A would be sensitive to radiation-induced myelotoxicity than vehicle controls. Six-week-old female B6C3F1 mice were injected intraperitoneally on alternate days over a week with a total dose of 0, 20 or 40 mg/kg of OCT A and bone marrow parameters monitored for up to 16 weeks. There was a suppression of marrow granulocyte- macrophage progenitors (CFU-C) in OCT A treated animals which returned to normal values by two weeks (20 mg/kg group) or by five weeks (40 mg/kg group) following the last treatment. Some of the OCT A treated mice were additionally irradiated with 200 rads whole body irradiation at 10 and 52 days following OCT A injections. Irradiation caused a significant reduction in CFU-C's in all mice but the effect were more pronounced in the mice that had received OCT A previously. The delayed recovery in bone marrow progenitors was also reflected in lower peripheral white blood counts after the second irradiation in 40 mg/kg OCT A treated mice as compared to the untreated irradiated controls. This indicated that residual bone marrow effect of OCT A makes the mice more sensitive to subsequent irradiation induced injury. These results were accepted for publication in Toxicol., 1988 (In press).